The production of elongate profiles from one or more mixtures of rubber or thermoplastics materials utilising an extrusion head to which the mixture or mixtures are supplied by one or more extrusion devices is known. It is also known to cool the profile by conveying it through a cooling zone comprising a plurality of conveyor belts. An apparatus used for producing a profile in the form of a tread strip from two different rubber mixtures is disclosed in German Patent Specification No. 2 201 631. In such prior specification, two separate screw presses, which are disposed one above the other, feed different rubber mixtures to an extrusion head where the material mixtures are combined and extruded therefrom in the form of a tread strip. U.S. Pat. No. 2,382,177 also shows such an arrangement.
The extruded tread strip is then conducted over a tread strip weighing device. Such weighing is achieved by weighing the amount of material located on a rotating conveyor belt. The desired weight of the tread strip is known and if the actual weight deviates therefrom, a certain degree of correction can be achieved. Thus, if the actual weight is higher than the desired weight, the rate of transfer of the tread strip can be increased. This effectively causes a weight reduction to be achieved because the strip is elongated.
In practice, therefore, such apparatuses for producing tread strips have been operated at discharge rate which is slightly in excess of a desired rate. The extruded tread strip is then brought to its desired weight as a result of the elongation.
The only regulation of the characteristics of the tread strip profile offered by such an arrangement is that the extruded tread strip can be elongated or compressed by increasing or reducing the rate of transfer of the strip. However, the rate at which the tread strip is discharged from the nozzle must be considered as the minimum rate for the tread strip transfer apparatus and, for the purpose of regulating the profile thickness, it is vital that the rate of transfer does not fall below this minimum rate.
U.S. Pat. No. 4,097,566 also discloses a method and apparatus for producing profiles formed from a plurality of mixtures. The method which is described therein is highly complex having regard to the regulating techniques employed and, additionally, has one fundamental disadvantage. If, for example, one of the weighing devices or one of the measuring devices used for measuring the width of a deflection loop ascertains that the produced profile does not have the desired dimensions, adjustment can still only be effected by either extending or compressing the profile strip.
Each extension or elongation operation on the profile strip, produced, for example, by accelerating the conveyor belts, provides the profile strip with the correct weight but with totally inaccurate and uncontrolled dimensions. This is evidenced by the uncontrolled and irregular longitudinal shrinkage which occurs when the profile strip passes through the cooling zone.
An additional disadvantage of such a known arrangement resides in the fact that the apparatus used is very long. Accordingly, from the time when the deviation from the desired weight of the profile strip is ascertained to the time of its correction, a large amount of unusable material has passed through the apparatus. This is because a large amount of profile strip passes between the particular measuring point and the source of error, that is to say, the point of deviation from the desired value. Such profile strip portion is, therefore, useless.
Basically, it must be stressed that it is not feasible to extend an extruded profile to compensate for variations in thickness of the profile because each extension operation produces, during cooling, longitudinal shrinkage which is too inaccurate and too uncontrollable.